Communication devices such as terminals are also known as e.g. User Equipments (UE), wireless devices, mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The development of the future 5th Generation (5G) access technology and air interference is still very premature but there have been some early publications on potential technology candidates. A candidate on a 5G air interface is to scale the current LTE, which is limited to 20 Mega Hertz (MHz) bandwidth, N times in bandwidth with 1/N times shorter time duration, here abbreviated as LTE-Nx. A typical value may be N=5 so that the carrier has 100 MHz bandwidth and 0.1 millisecond slot lengths. With this scaled approach, many functions in LTE can be re-used in LTE-Nx, which would simplify standardization effort and allow for a reuse of technology components.
The carrier frequency for an anticipated 5G system may be much higher than current 3rd Generation (3G) and 4th Generation (4G) systems, values in the range 10-80 Giga Hertz (GHz) have been discussed. At these high frequencies, an array antenna must be used to achieve coverage through beamforming gain, such as that depicted in FIG. 1.
FIG. 1 depicts a 5G system example with three Transmission Points (TP): TP1, TP2 and TP3, and a UE. Each TP utilizes beamforming for transmission. The beams are represented with white lobes in the figure.
Since the wavelength is less than 3 centimeters (cm), an array antenna with a large number of antenna elements may be fit into an antenna enclosure with a size comparable to 3G and 4G base station antennas of today. To achieve a reasonable link budget, a typical example of a total antenna array size is comparable to an A4 sheet of paper.
The beams are typically highly directive and give beamforming gains of 20 decibels (dB) or even more since so many antenna elements participate in forming a beam. This means that each beam is relatively narrow in horizontal and/or azimuth angle, a Half Power Beam Width (HPBW) of 5 degrees is not uncommon. Hence, a sector of a cell may need to be covered with a large number of potential beams. Beamforming may be seen as when a signal is transmitted in such as narrow HPBW that it is intended for a single wireless device or a group of wireless devices in a similar geographical position. This may be seen in contrast to other beam shaping techniques, such as cell shaping, where the coverage of a cell is dynamically adjusted to follow the geographical positions of a group of users in the cell. Although beamforming and cell shaping use similar techniques, i.e., transmitting a signal over multiple antenna elements and applying individual complex weights to these antenna elements, the notion of beamforming and beams in the embodiments described herein relates to the narrow HPBW basically intended for a single wireless device or terminal position.
Here, a system with multiple transmission nodes may be considered, where each node has an array antenna capable of generating many beams with small HPBW, such as that of FIG. 1. These nodes may then for instance use one or multiple LTE-Nx carriers, so that a total transmission bandwidth of multiples of hundreds of MHz may be achieved leading to downlink peak user throughputs reaching as much as 10 Gigabytes (Gbit/s) or more.
Multi-Antenna Techniques
Multi-antenna techniques may significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a Multiple-Input Multiple-Output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
One of the important characteristics of the channel conditions in the field of high rate multi-antenna transmission is the so-called channel rank. Roughly speaking, the channel rank may vary from one up to the minimum number of transmit and receive antennas. Taking a 4×2 system as an example, i.e., a system with four transmitter antennas and two receive antennas; the maximum channel rank is two. The channel rank may vary in time, as the fast fading alters the channel coefficients.
The LTE standard has MIMO support. A core component in LTE is the support of MIMO antenna deployments and MIMO related techniques. In LTE-Advanced there is enhanced support of up to 8-layer spatial multiplexing for 8 Transmission (Tx) antennas with an enhanced channel dependent precoding. The precoding is aimed for high data rates in favorable channel conditions and is especially targeting cross-polarized antenna setups. An illustration of the spatial multiplexing operation is provided in FIG. 2, which depicts a transmission structure of precoded spatial multiplexing mode in LTE, where r layers to be transmitted to one UE, of which a code word may be mapped to more than one layer in general, are precoded with a precoding matrix with r input and N_T outputs, one per transmit antenna port from the eNB. Each of the N_T streams of precoded modulation symbols are then converted to a time domain signal using the inverse Fast Fourier Transform (IFFT). The up to 8 Tx antennas are assumed to be co-located, that is, placed at the same eNB site. This means that the UE may use the channel from any, or all, of the Tx antennas to estimate, e.g., Doppler parameters or delay spread since they are equal.
As can be described by expression (1) and also illustrated in FIG. 2, the information carrying symbol vector s is multiplied by an NT×r precoder matrix WNT×r, which serves to distribute the transmit energy in a subspace of the NT, corresponding to NT antenna ports, dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a Precoder Matrix Indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. If the precoder matrix is confined to have orthonormal columns, then the design of the codebook of precoder matrices corresponds to a Grassmanian subspace packing problem. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved since multiple symbols may be transmitted simultaneously over the same Time/Frequency Resource Element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the DL, and Discrete Fourier Transform (DFT) precoded OFDM in the UL, and hence the received NR×1 vector yn for a certain TFRE on subcarrier n, or alternatively data TFRE number n, is thus modeled byyn=HnWNT×rsn+en  (1)
where en is a noise/interference vector obtained as realizations of a random process. The precoder, WNT×r, may be a wideband precoder, which is constant over frequency, or frequency selective, where a new precoder may be used for each sub-band or block of resource blocks.
The precoder matrix WNT×r is often chosen to match the characteristics of the NRxNT MIMO channel matrix H, resulting in so-called channel dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced. Each column in WNT×r equals the precoding vector used for one layer, according to expression (1). For example, the first element in vector sn which represents a symbol transmitted on the first layer, is precoded using the first column on WNT×r according to the matrix-vector multiplication.
In closed-loop precoding for the LTE DL, the UE transmits, based on channel measurements in the forward link DL, recommendations to the eNodeB of a single suitable precoder WNT×r to use, in some feedback modes one precoder per subband. This is called Channel State Information (CSI) reporting. The reporting from the UE is constrained to a codebook, but the transmission from the eNodeB may or may not be constrained to a codebook. The former case corresponds to so-called codebook based transmit precoding on the transmit side and is usually associated with the use of Common Reference Signals (CRS) for demodulation. The case when the transmissions are not constrained to a precoder codebook usually relies on DeModulation Reference Signals (DMRS) for demodulation and is sometimes referred to as non-codebook based transmit precoding.
A single precoder that is supposed to cover a large bandwidth, wideband precoding, may be fed back in the Channel State Information (CSI) report. It may also be beneficial to match the frequency variations of the channel and instead feed back a frequency-selective precoding report, e.g. several precoders, one per subband. This is an example of the more general case of CSI feedback, which also encompasses feeding back other entities than precoders to assist the eNodeB in subsequent transmissions to the UE. Such other information in the CSI report may include Channel Quality Indicators (CQIs) as well as a transmission Rank Indicator (RI).
For the LTE uplink, the use of closed-loop precoding means the eNodeB is selecting precoder/s and transmission rank and thereafter signals the selected precoder that the UE is supposed to use.
The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. For efficient performance, it is important that a transmission rank that matches the channel properties is selected. Often, the device selecting precoders is also responsible for selecting the transmission rank—one way is to simply evaluate a performance metric for each possible rank and pick the rank which optimizes the performance metric. These kinds of calculations are often computationally burdensome and it is therefore an advantage if calculations can be re-used across different transmission ranks. Re-use of calculations is facilitated by designing the precoder codebook to fulfill the so-called rank nested property. This means that the codebook is such that there always exists a column subset of a higher rank precoder that is also a valid lower rank precoder.
A common antenna setup is to use dual polarized antennas at the eNodeB where antennas with same polarizations are closely spaced, between 0.5 and 1 wavelength. Polarized antennas significantly reduce the total size of the antenna array used in multi-layer MIMO applications since the channels from different polarizations have low correlation, and are thus suitable for MIMO transmission. For best MIMO performance, channels from different antennas may have low correlation, while for best beamforming performance, channels from different antennas may instead have high correlation. In case of a dual polarized antenna setup, both are achieved, the closely spaced antennas of the same polarization have high correlation, thus suitable for a polarized beam. On the other hand, the closely spaced antennas of the other polarization, which are suitable for a second beam, have likely low correlation compared to the channel of the first polarization beam, although the average channel gain of the two polarization beams are equal, if the beams are pointed in the same direction.
FIG. 3 depicts a dual polarized antenna array with four antenna elements of −45 degree slanted polarization, represented as solid bars, and a second array of four antenna elements with +45 degrees slanted polarization, represented as dashed bars. Each polarization has a precoder that maps the transmitted bits on to the four antenna elements, i.e. four transmit antenna ports, where the phase and possibly also amplitude precoding weights is applied for each antenna element to form a polarization beam.
An LTE codebook targeting the dual polarized transmit antenna setup was introduced in Rel-10 for 8 Tx eNodeB and in Rel-12 for 4 Tx eNodeB. In this codebook, a codebook of precoding matrices {tilde over (w)} with smaller than the full dimension of the antenna array was introduced, targeting a co-polarized antenna group only. Hence, the precoding matrix {tilde over (w)} corresponds to a polarization precoder in FIG. 3, which is illustrated as the precoder for polarization 1 and the precoder for polarization 2, respectively. Since the correlation is high within the antenna group having the same polarization, due to the narrow antenna spacing, it may be appropriate to use a grid of beam codebook implemented from DFT based precoder vectors for {tilde over (w)}. This precoder may also be known as the inner precoder.
The outer precoder, W(t), adjusts the relative phase shift between the precoded polarizations. For rank 1, the precoder may for example be formed as:
                              W          =                                                    [                                                                                                    w                        ~                                                                                    0                                                                                                  0                                                                                      w                        ~                                                                                            ]                            ⁢                              w                                  (                  t                  )                                                      =                                          [                                                                                                    w                        ~                                                                                    0                                                                                                  0                                                                                      w                        ~                                                                                            ]                            ⁡                              [                                                                            1                                                                                                  α                                                                      ]                                                    ,                                  ⁢                  α          ∈                      {                          1              ,                              -                1                            ,              j              ,                              -                j                                      }                                              (        2        )            
As shown here, the tuning precoder W(t)=[1α]T adjusts the phase between a first and a second group of antennas. Also, the same inner precoder {tilde over (w)} is used for both polarizations in this example. In this case, the first and second groups correspond to the upper and lower halves, respectively, of the rows of the precoder W.
FIG. 4 illustrates transmission of a code word over a dual polarized antenna setup in LTE, where the UE selects the DFT based polarization precoder vectors for {tilde over (w)}, and the co-phasing angle alpha (α). Hence, the same code word is transmitted in the two polarization beams but one of them is multiplied with a co-phasing term alpha so that the two code word copies are coherently combined in the receiver.
The rank 2 case follows similarly in the LTE codebook as:
                              W          =                                    [                                                                                          w                      ~                                                                            0                                                                                        0                                                                              w                      ~                                                                                  ]                        ⁡                          [                                                                    1                                                        1                                                                                        α                                                                              -                      α                                                                                  ]                                      ,                                  ⁢                  α          ∈                      {                          i              ,              j                        }                          ,                            (        3        )            
See FIG. 5 for the corresponding description of this rank 2 precoder. FIG. 5 illustrates transmission of two code words over a dual polarized antenna setup in LTE, where the UE selects the DFT based polarization precoder vectors for W and the co-phasing angle α. Hence, the same code word is transmitted in the two polarization beams but one of them is multiplied with the co-phasing term a so that the two code word copies are coherently combined in the receiver. The second code word is transmitted in the same beams, but with the co-phasing angle −α.
Codewords and Codewords to Layer Mapping
Modern wireless communication systems targeted for packet based communication often include Hybrid Automatic Retransmission reQuest (HARQ) functionality on the physical layer to achieve robustness against the impairments of the radio channel. LTE and Wideband Code Division Multiple Access (WCDMA) are two examples of systems in which such functionality is available. The basic idea behind HARQ is to combine Forward Error Correction (FEC) with ARQ by encoding the information containing data block and then adding error-detection information such as Cyclic Redundancy Check (CRC). After reception of the coded data block, it may be decoded and the error-detection mechanism may be used to check whether the decoding was successful or not. If the data block was received without error, an ACKnowledgment (ACK) may be sent to the transmitter indicating successful transmission of the data block and that the receiver may be ready for a new data block. On the other hand, if the data block was not decoded correctly, a Negative ACKnowledgment (NACK) may be sent meaning that the receiver expects a retransmission of the same data block. Subsequent to the reception of the retransmission, the receiver may choose to either decode it independently or utilize some or all previous receptions of the same data block in the decoding process.
The encoded bits originating from the same block of information bits is referred to as a codeword. This is also the terminology used in LTE to describe the output from a single HARQ process serving a particular transport block and comprises turbo encoding, rate matching, interleaving etc. The codewords are then modulated and distributed over the antennas.
Precoding, as described in the previous sections, is a popular technique used in conjunction with multi-antenna transmission. The basic idea is to mix and distribute the modulation symbols over the antenna while possibly taking the current channel conditions into account. This is often realized by multiplying the information carrying symbol vector by a matrix selected to match the channel. The symbol vector may contain modulation symbols from potentially all the codewords and the codewords thus map to a sequence of symbol vectors. These sequences form a set of parallel symbol streams and each such symbol stream is referred to as a layer. Thus, depending on the precoder choice, a layer may directly correspond to a certain antenna or it may, via the precoder mapping, be distributed onto several antennas.
In a multi-antenna system, often referred to as a MIMO system, it may be appropriate to transmit data from several HARQ processes at once, also known as multi-codeword transmission. Depending on the channel conditions, this may substantially increase the data rates since in favorable conditions the channel may roughly support as many codewords as the minimum of the number of transmit and receive antennas. However, in LTE, the maximum number of code words is two, irrespectively of the number of transmit and receive antennas.
The channel rank, as described earlier, determines how many layers, and ultimately also codewords, may be successfully transmitted simultaneously. In conjunction with precoding, adapting the transmission to the channel rank involves using as many layers as the channel rank. In the simplest of cases, each layer may correspond to a particular antenna. The issue then arises of how to map the codewords to the layers. Taking the 4 transmit antenna case in LTE as an example, the maximum number of codewords is limited to two as mentioned above, while up to four layers may be transmitted. A fixed rank dependent mapping according to FIG. 6 is used. FIG. 6 illustrates a codeword to layer mapping for a four antenna system with precoding according to the description in LTE standard documents 3GPP TS 36.211, TS 36.212 and TS 36.213. For a single rank transmission, a first code word (CW1) is mapped to one layer and precoded with a precoding vector to map the precoded code word to all four transmit antenna ports. For rank 2, two code words (CW) are used, so there are two inputs to the precoder and four outputs. For rank 3, the second CW, CW2, has the double number of modulated symbols compared to CW1. This CW is then Serial to Parallel (S/P) converted into two layers, so CW1 is mapped to layer 1 and CW2 is mapped to layer 2 and layer 3 in LTE. All layers have the same number of modulated symbols. Since the number of CW is maximally two in LTE, there is also a S/P for CW1 in the rank 4 case. In rank 5 case, which is not shown in FIG. 6, there is a S/P operation for CW2 that splits CW2 into three layers, in total five layers. This principle holds up to 8 layer transmission, which is the maximum in LTE. If there is a retransmission of a CW, CWn, where a S/P has been used in the previous, e.g., first, transmission of this CW, for example CW2 in a rank 3 transmission, then the S/P is used also in a rank 2 retransmission, which is shown in the lower left part of FIG. 6.
This also means that the first column of the precoding matrix determines the precoder for Code-Word 1 (CW 1) in a rank 1 transmission. For a rank 2 transmission, the second column of the precoding matrix determines the precoder for CW 2. Since there are at most 2 code words transmitted, it means that for rank 3 transmission, CW 1 is using the first column of the precoding matrix while CW2 is using column 2 and 3. Finally, for a rank 4 transmission, CW 1 may use column 1 and 2 whereas CW 2 may use column 3 and 4 of the precoding matrix.
If 8 Tx antennas are used, and up to 8 layer transmission is possible, then the same principle applies where for rank 5, CW 1 use column 1, 2 and CW 2 use column 3, 4, 5 and for rank 6 CW 1 use column 1, 2, 3 and CW 2 use column 4, 5, 6 and so on. FIG. 7 illustrates a rank 8 example of LTE codeword to layer mapping. Here, y is a N_T by 1 vector containing the precoded signals. W is the precoding N_T times r matrix, and x is the rx1 vector containing the r layers. The transmitted signal per antenna port is the matrix-vector product Wx. W1 . . . w8 are the N_T times 1 precoding vectors, i.e., the beamforming weight vectors, for layer 1 . . . 8, respectively. X1 . . . x8 is thus the modulated symbol transmitted on layer 1 . . . 8, respectively. In this figure, x1 . . . x4 are coming from the same encoder output, i.e. they belong to the same Code Word 1 (CW1). And x5 . . . x8 come from Code Word 2 (CW2). This means that the precoder for CW1 is the sub-matrix obtained by the columns [w1 . . . w4], and the precoder for CW2 is the sub-matrix obtained by the columns [w5 . . . w8].
To achieve high rank transmission in LTE, at lower carrier frequencies, a rich scattering channel is needed, sometimes characterized in the eigenvalue spread, ratio of the largest to the smallest eigenvalue, of the MIMO channel H. When the spread is close to one, the channel is rich in MIMO sense and precoders can be directed in any direction and still have very good channel gain. Hence, in this rich scattering environment, the benefit of precise precoding is reduced when rank is increased. This is reflected in the codebook design for the 8Tx LTE codebook, where fewer precoding matrix candidates exist for higher rank, and for rank 8, there is only a single precoding matrix, hence there may be no need for PMI feedback if rank 8 is signaled.
Existing methods for channel state feedback from a receiver to a transmitter are based on a number of assumptions, such as a rich scattering channel as just described, which are applicable to lower carrier frequencies of transmission than those expected to be used in future systems. These assumptions are no longer valid at higher carrier frequencies of transmission, which makes the existing methods for channel feedback inadequate for future transmission systems.